1. Field of the Invention
This invention generally relates to video display systems, and, more specifically, relates to a hardware cursor for a video controller that outputs the cursor on the display monitor without requiring the control software to perform all the computations and data manipulations for the cursor.
2. Description of the Related Art
Most display systems comprise a Video Controller coupled to Video RAM and to a RAMDAC. The RAMDAC converts the digital display information to appropriate analog output voltages for a display monitor. The Video RAM stores the digital display data which the Video Controller outputs to the RAMDAC. In conventional display systems the cursor for the display is generated by the control software for the Video Controller, which keeps track of the location of the cursor, and performs the required manipulations on the display data to make the cursor appear on the display monitor. This software implementation of the cursor creates overhead for the Video Controller software, since it must perform the cursor calculations and store the resultant data for each scan of the display.
Recently some manufacturers of display electronics have implemented a hardware cursor which eliminates this software overhead for the Video Controller by implementing the cursor functions in hardware. One such example is found in the Bt485 RAMDAC by Brooktree Corporation. This RAMDAC has a 64.times.64.times.2 Cursor Memory, with Cursor Control Circuitry which automatically outputs the Cursor Data in the Cursor Memory when the scan to the RAMDAC reaches the current cursor location. This frees up the Control Software of the Video Controller from having to perform all the data manipulations for the cursor during each scan. However, this implementation requires 1 Kbyte of RAM on the RAMDAC for the Cursor Memory. This much RAM takes up substantial silicon and increases the complexity of the RAMDAC chip.
Western Digital has also implemented a hardware cursor in their WD90C31 VGA Controller Chip. This implementation is similar to that used by Brooktree, having a 64.times.64.times.2 Cursor Memory with Cursor Control Circuitry to perform essentially the same function as the Brooktree part. The primary difference is that the Cursor Memory is not implemented on the WD90C31 chip. Instead, an unused portion of the Video RAM is used as Cursor Memory. This design provides the flexibility of a hardware cursor without the penalty of additional memory to store the cursor information.
The specific design and configuration of the Western Digital part is unknown other than general specifications published in the WD90C31 VGA Controller data sheet (Advanced Information, Aug. 7, 1991). A simple design implementing the functions of a hardware cursor which uses an unused portion of display memory for its Cursor Memory would allow efficient implementation in silicon with a minimum number of gates.
Therefore, there existed a need to provide a hardware cursor that relieves the Control Software from computing and outputting the cursor during each scan, that does not require additional RAM to store the cursor information, and that has a simple design which minimizes the number of gates and hence, the silicon used for its implementation.